1. Field of the Invention
This invention relates to the conversion of heavy hydrocarbon feedstocks to produce increased proportions of motor fuel range hydrocarbons and fuel oils. More particularly, this invention relates to converting a heavy hydrocarbon feedstock to liquid hydrocarbon products by contacting said feedstock with an alkali metal carbonate molten medium and hydrogen. Still more particularly, this invention relates to the conversion of a heavy hydrocarbon feedstock such as atmospheric and vacuum residua, crude oils and the like, at elevated pressures and in the presence of hydrogen in a regenerable molten medium containing boron oxide and an alkali metal carbonate to produce predominantly liquid hydrocarbon products such as a gas oil and carbonaceout materials. At least a portion of the carbonaceous materials formed during the cracking process are gasified by contacting said carbonaceous materials in the molten medium with air, steam or carbon dioxide at elevated temperatures in order to regenerate the melt.
2. Description of the Prior Art
Heavy hydrocarbon materials such as atmospheric or vacuum residua, crude oil and the like, are typically subjected to a viscosity-reducing or "visbreaking" treatment at high temperatures and elevated pressures to convert, by a mild thermal cracking, the feedstock to about 5 to 15% gas oil, about 5 to 15 volume % gasoline, and about 75 to 85% heavy fuel oil. The specific temperatures, pressures, and feed rates employed in the visbreaking process depend upon the type of visbreaker feed. The gas oil formed by such a process represents a feedstock suitable for the production of additional amounts of high quality gasoline by catalytic cracking or, after suitable finishing, such as sulfur and/or nitrogen removal, an acceptable distillate fuel or lube oil fraction.
The conversion of heavy hydrocarbon feedstocks, such as residua, is relatively difficult in view of their tendency to form coke when subjected to moderately high temperatures. This coke-forming tendency has also limited the industrial application of molten heat transfer media in order to effect the hydrocarbon conversion of such feedstocks. The primary difficulty encountered when employing molten media systems for such conversion processes is that the carbonaceous particles, i.e., coke, produced during the conversion operation are not suspended in the melt, but form a separate phase which contaminate the liquid and gaseous products. With melts that partially suspended the coke, such as alkali metal halide eutectics, e.g., lithium-potassium chloride, the buildup of such carbonaceous materials in or above the molten medium necessitates additional steps to physically remove the carbonaceous particles from the melt.
It has been suggested that hydrocarbon feedstocks can be cracked in molten alkali metal carbonate, alkali metal hydroxide, or a mixture thereof, to form various hydrocarbon products and the molten medium thereafter regenerated by contacting the same with oxygen or steam (see U.S. Pat. Nos. 3,553,279; 3,252,774; German DT-OS 2,149,291; U.S. Pat. Nos. 3,505,018; 3,252,773; 3,438,727; 3,647,358; 3,438,728; Oil and Gas Journal, Sept. 27, 1971; U.S. Pat. Nos. 3,438,733; 3,434,734; 3,516,796; 3,551,108 and 3,647,358. Further, in Czechoslovakian Patent 109,952 it is disclosed that various compositions can be employed in the thermal cracking of hydrocarbons. While alkali metal carbonate based melts tend to absorb or disperse the coke formed in the conversion operation, the extent of coke dispersion is relatively low. This limited coke dispersion in the molten medium may cause process difficulties in a commercial environment.
Recently, it has been proposed to crack a hydrocarbon feedstock in a regenerable molten medium containing an alkali oxide in combination with a glass-forming oxide such as an oxide of boron (see U.S. Pat. No. 3,850,742). Such a molten medium, while exhibiting sufficient coke dispersion, suffers from the disadvantage of being corrosive in nature, thereby resulting in a significant materials of construction problem.